1. Field of the Invention
The present invention relates to a chain tension structure for imparting a tension to a chain which transmits a drive force.
2. Description of Background Art
A conventional chain tension structure is known wherein the chain tension structure is used for adjusting a tension of a chain by moving an axle of a rear wheel. See, for example, JP-A-5-105179.
FIG. 3(a) to FIG. 3(c) of JP-A-5-105179 are explained in conjunction with FIG. 22(a) to FIG. 22(c) of the drawings wherein the numerals have been renumbered.
FIG. 22(a) to FIG. 22(c) are explanatory views showing the conventional chain tension structure.
FIG. 22(a) is a cross-sectional view of one rear arm member 302 out of a pair of left and right rear arm members that vertically swingably supports a wheel by way of a rear axle 301 from a side. The rear arm member 302 includes an axle holder 303 for supporting the rear axle 301 on the inside thereof with an end portion of a bolt 304 mounted on the axle holder 303 which is allowed to penetrate a rear end of the rear arm member 302. A first nut 306 and a second nut 307 are threadedly engaged with an end portion of the bolt 304. A scale 308 is formed on a side surface of the axle holder 303 which is used as a reference for adjusting the position of the rear axle 301.
FIG. 22(b) is a lateral cross-sectional view of the rear arm member 302, wherein rear arm member 302 has a rectangular closed cross-sectional structure and the axle holder 303 is arranged on the inside of the rear arm member 302.
FIG. 22(c) is a view which observes a cross-section of the rear arm member 302 from above, wherein elongated holes 309, 309, which are formed in the longitudinal direction of the rear arm member 302 in an elongated manner, are provided on an outer wall and an inner wall of the rear arm member 302. The bolt-like rear axle 301 is allowed to penetrate the elongated hole 309 formed in the outer wall, an inner hole 311 formed in the axle holder 303, the elongated hole 309 formed in the inner wall and the wheel.
A sprocket is integrally mounted on the wheel and a chain is wound around the sprocket wheel.
To adjust a tension of the above-mentioned chain, a nut not shown in the drawing which is threadedly coupled to an end portion of the rear axle 301 is slackened, subsequently, the second nut 307 is slackened, and the first nut 306 is rotated. Due to such an operation, the rear axle 301 is moved together with the axle holder 303 along with the elongated holes 309, 309, and the axle-side sprocket wheel is also integrally moved with the rear axle 301 whereby the tension of the chain can be adjusted.
In the above-mentioned technique, since the rear axle 301 is moved to adjust the tension of the chain, a distance between an axle of another wheel (front wheel) of a bicycle and the rear axle 301 (that is, a wheelbase) is changed. Therefore, for example, a possibility exists wherein the turning performance of the bicycle is influenced.
Further, in addition to the adjustment of the rear axle 301 on the rear arm member 302 side, it is necessary to perform the adjustment of the rear axle 301 also on another rear arm member side in the same manner. Further, depending on the degree of tension of the chain, it may be necessary to repeat the adjustment on both rear arm members several times. Thus, the improvement of the adjusting operation can be enhanced.
In addition, a conventional drive force transmission unit structure for a vehicle, is known wherein the drive force transmission unit structure is provided with a freewheel, that is, a one-way clutch. See, for example, JP-UM-59-116640.
FIG. 1 of JP-UM-59-116640 is explained in conjunction with FIG. 24 of the drawings wherein the numerals have been renumbered.
FIG. 24 is a cross-sectional view showing the conventional drive force transmission unit structure of a vehicle, wherein a ratchet ring 303 is rotatably mounted on a body 301 of the freewheel by way of a plurality of steel balls 302. In addition, ratchet pawls 304 are raised tiltably in the substantially radial direction from an inner peripheral surface of the ratchet ring 303 to an outer peripheral surface of the body 301 of the freewheel so as to engage distal ends of the ratchet pawls 304 with ratchet teeth which are mounted on the outer peripheral surface of the body 301 of the freewheel thus connecting the sprocket wheel 306 to the ratchet ring 303 by a thread coupling.
Due to such a construction, the rotation of the sprocket wheel 306 in one direction with respect to the body 301 of the freewheel is allowed, while the rotation of the sprocket wheel 306 in another direction with respect to the body 301 of the freewheel is prevented. More specifically, the body 301 of the freewheel and the sprocket wheel 306 are integrally rotated. A ball pusher 307 is threadedly coupled to the body 301 of the freewheel to support the steel balls 302.
When the vehicle, that is provided with the above-mentioned freewheel, is provided with a multi-stage transmission mechanism which mounts a plurality of sprocket wheels on the ratchet ring 303 besides the sprocket wheel 306, to perform the transmission, for changing the winding of the chain around the sprocket wheel 306 or other sprocket wheel, it is necessary to apply a load to the chain from a sideward direction in a state wherein the chain is rotated. Accordingly, when the chain is not rotated, the winding of the chain cannot be changed to respective sprocket wheels. Thus, the transmission cannot be performed.
When the above-mentioned freewheel is mounted on a rear wheel of a bicycle, that is, a drive wheel, the sprocket wheel 306 is rotated by pedaling by way of the chain and a drive force is transmitted to the ratchet ring 303 from the sprocket wheel 306 by way of the ratchet pawls 304. Thus, the rear wheel which is mounted on the ratchet ring 303 is rotated.
When a rider travels on a slope or the like using inertia of the bicycle while placing his/her feet on the pedals without pedaling, only the rear wheel is rotated and the chain is not rotated whereby the transmission is impossible.
Accordingly, by adopting a structure which integrally connects the ratchet ring 303 to the body 301 of the above-mentioned freewheel, when the bicycle travels with inertia, the body 301 of the freewheel and the sprocket wheel 306 are integrally rotated along with the rotation of the rear wheel, whereby the chain is also rotated thus allowing the transmission. However, since the drive force transmission unit such as the pedal side, the chain, the freewheel and the like is always directly connected, it is preferable to provide an overload prevention mechanism to cope with a case in which a large load is applied to the drive force transmission unit.